Frequency variation response circuits



June 21, 1938. s. w. SEELEY 2,121,103

FREQUENCY VARIATION RESPONSE cIRcuITs Filed Oct. 17, 1935 3 Sheets-Sheet 2 AAAAAAA ff BY W ATTORNEY.

n I 19 70 A. /-t

NHWORK' June 21, 1938. s. w. SEELEY FREQUENCY VARIATION RESPONSE CIRCUITS Filed Oct. 17, 1955 3 Sheets-Sheet 5 INVENTOR. STUART w. SEELEY BY 4 7 -u-QA/ ATTORNEY.

Patented June 21, 1938 UNITED STATES PATENT OFFICE 2,121,103 FREQUENCY VARIATION RESPONSE cmo UITS

tion of Delaware Application October 17, 1935, Serial No. 45,413

18 Claims. (Cl. 250-20) My present invention relates to high frequency variation response circuits, and more particularly to frequency response networks of a type utilizing changes in phase relations of primary and secondary circuit' voltages which occur in coupled tuned circuits when the applied high frequency energy departs fromresonance with the tuned circuits.

In the past frequency variation response circuits have been proposed for many uses. Such uses have involved the indication of frequency departure from a predetermined frequency value; the maintenance of a resonant circuit at a frequency of a predetermined magnitude; or the utilization of signal energy in a receiver for automatically tuning a resonant circuit to a predetermined frequency. Such proposed circuits have usually employed frequency discriminator networks embodying resonant circuits mistuned by equalfrequency values to opposite sides of a predetermined operating frequency. The direct current component of the rectified output of a discriminator network is utilized, in such prior proposed circuits, to perform the functions referred to above. One of the chief disadvantages of such prior discriminator networks has been the difficulty in aligning the mistuned circuits of the discriminator with the resonant circuits of the receiving system'which are tuned to the operating frequency.

Accordingly, it may be stated that it is one of the primary objects of my present invention to provide a high frequency variation response network wherein there is not employed side circuits tuned above and below a predetermined center frequency, but wherein the frequency discrimi-' tern is greatly simplified, and signaling systems embodying such discriminator networks are rendered highly practical.

The aforementioned proposals of the prior art have all substantially utilized mistuned circuits in the discriminator network in such a manner that the differential direct current output of the mistuned circuits was obtained by virtue of alternate operation of rectifiers coupled to the mistuned circuits. That is to say in such prior circuits each mistuned circuit of the discriminator network is connected to a rectifier, and as the applied frequency departs from resonance with the desiredoperating frequency, the center frequency of the mistuned circuits, either one or the other of the mistuned rectifiers becomes operative to derive a direct current from the applied signal energy.

It is, then, one of the important objects of my present invention to provide a frequency variation response network which functions in an entirely different manner from such prior proposed circuits, and wherein there is established in the frequency response network a predetermined phase difference between a primary and secondary potential of a tuned network embodying primary and secondary circuits, the phase angle between the primary and secondary potentials varying as the applied energy varies in frequency from resonance.

Another important object of the present invention is to provide a method of, and apparatus for, obtaining differential direct current potentials, or currents, whose magnitude and polarity are determined by the amount and the sign, re-

spectively, of the difference between an applied frequency and a certain fictitious frequency, and wherein the action depends upon the fact that a 90 phase difference exists between the primary and secondary potentials of a double tuned transformer when energy of the resonant frequency is applied, and that this phase angle varies as the applied frequency varies.

An additional object of the invention is to provide a frequency discriminator network wherein tor sum potentials of the primary and secondary voltages may be realized, one maximizing above and one maximizing below the center frequency, which latter frequency is the common resonant frequency of the primary and secondary circuits, and rectifiers being utilized to rectify those sum voltages in such a manner that the resulting direct current voltages are added in opposition; thus the sum of the direct current voltages will be zero at resonance, and the sum will be some real value whose polarity will depend upon the sign of the frequency departure when the applied frequency departs from resonance.

Still other objects of the invention are to utilize the frequency variation response network of the present invention in the demodulator stage of a system adapted to receive amplitude-modulated, or frequency-modulated, carrier waves, and wherein the demodulator networks are not only adapted to produce voltages corresponding to the modulation voltages, but also produce direct current voltages to regulate the gain of carrier wave amplifiers, and the frequency of the local oscillator of the receiving system, when the latter is of the superheterodyne type.

Still \other objects of the invention are to improve generally the simplicity and efllciency of high frequency variation response networks, and more especially to provide such networks in a simple and economical manner which will not only be reliable in operation, but readily manu- I ment for analyzing the fundamental principle underlying the invention,

Fig. 2 illustrates a frequency discriminator network embodying a practical form of the inven tion,

Fig. 3 graphically illustrates the mode of ation of the arrangement in Fig. 2,

Fig. 4 is a circuit diagram of a superheterodyne receiver embodying the invention,

Fig. 5 is a modification of the demodulator network of the receiver of Fig. 4 when employed to receive frequency-modulated carrier waves,

Fig. 6 shows still another application of the present invention.

Referring now to the accompanying drawings, wherein like reference characters in the different figures designate similar circuit elements, there is shown in Fig. 1a circuit arrangement for analyzing, and visually indicating, the fundamental principle underlying the present invention. As stated heretofore, the functioning of the present invention depends upon a predetermined phase relationship which exists between the potentials of coupled tuned circuits. In particular, the action depends upon the fact that when a pair of resonant circuits are coupled, and each circuit is tuned to the same operating frequency, then a 90 phase difference exists between the potentials across the coupled circuits. As a result the phase angle between these potentials varies as the frequency of the energy applied to the coupled circuits departs from resonance therewith.

In Fig. 1 there is shown a pair of coupled resonant circuits P and S; the circuit P is tuned to a desired frequency by shunt condenser I, while circuit S is tuned to the same frequency by condenser 2. The high frequency waves which are to be applied to the double tuned network PS are derived from a source 3 of high frequency waves; and the source may be, for example, a signal generator capable of generating waves having a frequency of about 465 k. 0. Such a source includes a device enabling it to be adjusted in frequency so that the frequency of the waves can be varied; and those skilled in the art are fully aware of such devices. An amplifier 4 is ,used to amplify the waves from source I prior to impression upon circuit P. The numeral 5 designates an oscilloscope of a well known type; the deflector plates being denoted by numeral 0, and the fluorescent screen thereof bearing numeral 1.

To visually depict the relations between the voltages across primary and secondary circuits P and 8 oil and on resonance with the impressed waves from source I, a pair of the plates I are connected across circuit P, while the other pair of plates 6 are connected across circuit S. Assume,

now, that each of circuits P and S is tuned to a predetermined frequency of source 3, say 465 k. c., and waves of that frequency are impressed on amplifier 4 by source 3. A circular pattern 8 will form on the screen 1. This circle was observed to increase, or decrease, in diameter as the amplitude of the waves from source 3 increased, or decreased, respectively. Again, as the frequency of the waves generated by source 8 varied, the shape of pattern 8 was observed to change. The dotted ellipse 8 denotes the appearance of the pattern shape when the frequency of the impressed waves is varied. The degree of coupling between P and S determines whether or not the major axis of the ellipw will exceed the diameter of circle 8. Thus, if the coupling is adjusted to critical value, or over, the major axis will be greater as the applied frequency departs from resonance.

It will, therefore, be seen that the shape of the pattern on the screen 1 is dependent on the phase relations of the voltages applied to plates 6 by circuits P and S. Changes in amplitude ofi, or on, resonance only varies the size of the pattern. Further, a change of impressed frequency of! resonance with circuits P and S will result in an appreciable change in the form of the pattern 8. These relations arereadily understood when it is realized that a 90 phase difference exists between the potentials of circuits P and S when energy of the resonant frequency is applied, and that this phase angle varies as the applied frequency varies. The oscilloscope 5, then, demonstrates in a visual manner the effect of the applied frequency on the phase relation between the potentials across P and S, and proves that for applied frequencies other than the resonant frequency of circuits PS the voltages across these two circuits are not in time quadrature.

If, now, the primary and secondary voltages are added vectorially the absolute magnitude of the resultant vector will be greater on one side of resonance than on the other side. This vector sum can be physically realized, by way of example, if the circuits P and S are connected in tandem, applying the input potentials to one circult, and taking the output oil across both circuits in series. In this manner an action similar to that of a side circuit is produced even though the primary and secondary are both tuned to the center frequency. J

A more suitable manner of realizing the vector sum of the primary and secondary voltages is shown in Fig. 2. The primary tuned circuit is designated by numeral III, and is connected in the plate circuit of an amplifier II. A source I2 of high frequency waves is connected to impress such waves on the input electrodes of amplifier H. The secondary tuned circuit I3 is resonated to the frequency of circuit III. The high alternating potential side of circuit III is connected condenser IE to a center tap on" coll I l/of circuit 13, the coil M and coil 15 being magnetically coupled. The condenser l6 merely 'servesto isolate the direct current plate potential (from source, 3) of the primary circuit, and 'its'reactance is, small enough tobe disregarded as far as the frequency of operation is concerned.

nested-to the primary-circuit l0, twopotentials will be realized. One of these potentials Ezfmax'i- 'mizes above the center frequency fo (the resonant frequencyof both circuits l0 and I3); the other potential E1 maximizing below thecenter frequency. This is graphically represented in Fig. 3, wherein the relations, in scalor magnitudes, betweenEi, E2" and frequency are shown in solid line curves.

If a transformer, such aria-l4, is connected inthe mannerlshown in Fig. 2, and the frequency of .thefwaves from'source I2 is equal .to

" fc, then thetwo resulting output potentials will be equal injniag'nitude' If these potentials E1 andEzl are applied to two, separate, like detec- Itors, or frfectifiers ll l8, and the resulting direct currentfvoltages (or directcurrents) are added infopposition," the sum'will be equal to zero. .The ioutput load comprises resistors I 9 and 20, .of like magnitudal,connectedin-series between the cathodes of the diode 'rectifiers, a condenser 2|, of low impedance at the operating frequency}; being connected in shunt'with the load resistors". The junction of resistors 19 and 20 is connected to the centertap on the coil l4 through a radio frequency choke 22. Condenser 23, in shunt withresistor 20, acts with coil 22. to decrease the effect of the resistors on the Q [value of .the primary; the symbol Q being equalto 3' l I V w lithe" frequency o f'jt he waves'fiapplied from source l2 departs from jre'sonance'fthat is the I resonant frequency of each of circuits l0 and I3, the sum of the ,rectified outputs of these two circuits combined in opp ifiiQn will be some real value whose polarityjwill depend upon the ,sign of the frequency departure.

The dotted line curve in Fig.3 designates the difierence inscalor maginitrides-of' the potentials E; and E2, assuming the latter are rectified and added in oppositiong ItWill beobserved from 'Fig. 3 thatat 'th resonant f 'fiquer cy of the primary and s ecabove conditions. ,This does not mean thata larger. secondary with the same prlmary,or a different'valueot coupling, would not give a greater number of volts per cycle change in the primary plus one half the secondary sum, but in such event 5 the resultant itself wouldbegreater. Circuit, or other, requirements might necessitate an exceedingly low tuned primary impedance; in which case a much higher ratio would be in order.

It will thus be seen that there is shown in Fig. 2 an embodiment of the present invention, .wherein there is obtained differential direct current potentials (or currents) whose magnitude and polarity are determined by the amount and the sign, respectively, of the difierencebetween an applied frequency and a certalnfflctitious frequency. With regard to the sensltivityof the frequency variation response network shownin Fig. 2, a measure of the sensitivit may he the developed direct current volts, or amperes per cycle of frequency deviation, per volt applied'to' the grid of the tube whose plate circuit contains the primary l5. Regardless of the type of detectors employed this quantity will bea tunet vtion of the rate of change, with frequency, of the dii ference between the magnitudes of the input potentialsto-the two detectors ll and -I8, or the slope of curve E1 E2,( l ig. 3).11 these magni- .tudesv are plotted against frequency .difi erence (both positive'and negative) the curves willln- 'tersect on the zero abscissa-ordinate with slopes equal but opposite in sign, as shown in Fig. 3.

The slope of the curve representing their dif- 'ference is, therefore, equal to twice the slopeat- [,the center, frequency, .of the curve of input; po-

tentials to one of the detectors. This establishes thesignificance of afactor which will be termed S which equals two times the-first deriva- .tive with respect to frequency, at resonance,

an expression for absolute magnitude, of, input 43 potential to one of the detectors. S is the slope of the curve E1.E 2 in Fig. 3. 'It :is pointed out that the value of theordlnate atthe point of intersectionofjthe two curves Erand- E z becomes significant only when. detectors otherthan those 45 with linear characteristics are used. From theor'eticalconsiderations itcanj be 'shownj that S is independent of frequency, and. thatiamong other things it is' a function of the' secondary. tc'priniary inductance ratio, as well "as the ratio be- 1 Q tween actual and critical, couplingsv between .prilf d secondary. Further, it 'c'anTbe' demonstrated 'thatth e optimum value of coupling will be less than critical for any ratio of secondarvto primary inductance, The sensitivity. of the network can be made very great. roe example,l'w ith roper constants and with l'volt fromsourc'e fl appiiedit ispossible tqsecure an unb alance'of 1.13 volt 's' at f the dete or input points. iff j-the,

firms Zeigarnple nnstrate th order {sensitivities f-WlliGh ma b e obtained 'ifthe need"arises.'

. a maximum 5 983 at-resonance; th Ill8 1 1 0 willv normallwi'appeam,at positive :"and 5% negative values corresponding to frequencies directions on the two sides.

which are sufficiently well separated to give adequate operating range for most circuit applications of the response network. This is particularly true if the range of frequencies applied to the network is limited by the selectivity of preceding circuits.

If it becomes necessary to increase the frequency separation of the two maxima it may be done by either increasing the value of coupling above the optimum, or by decreasing the Q of the circuits. Either method will decrease the factor S at the center frequency, although an increase in coupling will cause the least change in sensitivity for a given increase in separation.

- If square law detectors are used, their outputs will be proportional to the square of the scalor magnitudes of the applied potentials. It can be shown that if square law detectors are used, the optimum coupling is independent of the ratio between secondary and primary inductances, and is equal to 0.578 times critical coupling.

The response network shown in Fig. 2 shows one specific manner for combining the direct current output potentials, or currents, of. the rectifiers I! and I 8 to produce the differential effect. It is to be clearly understood, however, that detectors of the plate rectification type may be used instead of the rectifiers -shown. In that case a differential winding would be placed between the two plates of the detector tubes; and the magnetic field of the differential winding will then be zero at resonance, and in opposite Thus, detected output currents would be addedv in opposition. The response network of Fig. 2 is capable of many circuit applications. The diodes l1 and i 8 need not be separate tubes, but may be disposed within a common tube envelope, as in the 6H6 type tube. Where the waves from source I 2 are modulated carrier frequencies, the condenser 2| not only has a low impedance at the operating frequency, but, in general, it is desirable that it be low at useful modulating frequencies.

The resistances of the series resistors l9 and 20 may be between 0.5 and 1.0 megohm, and it is further pointed out that the radio frequency choke coil 22 is optional. However, if this choke coil is used, then; is desirable that the condenser 23 be used. If the resonant, or center, frequency is applied to the grid of the amplifier tube H, the voltages E2 and E1 will be equal. These voltages are rectified by the diodes I1 and I8, and direct currents will flow into resistors 19 and 20 in opposite directions with respect to ground. Thus, the net direct current potential produced by the two voltage drops between the cathode side of resistor i9 and ground is equal to zero.

If, however, the applied frequency departs from resonance the potentials across the diodes will be unequal in magnitude. As a result unequal voltage drops will be produced in the resistors l9 and 20 anda direct current potential will exist across both resistors, the polarity of which net direct current potential will depend upon the sign of the frequency departure.

The network of Fig.2 is capable of many uses. Fig. 4 illustrates one such use wherein a superheterodyne receiver utilizes the response network for a triple function. The received signals are demodulated; automatic volume control (AVC) voltage is provided from the demodulated signals; and automatic local oscillator frequency control (AFC) voltage is also derived from the demodulated signals. The receiver is of a conv the broadcast range of 550 to 1500 k. c.

ventional type adapted to receive amplitudemodulatedcarrier waves; for example, those in The receiver may comprise the usual signal collector A followed by a tunable signal amplifier 30. The amplified signals are fed to a first detector Ii which has a tunable input circuit 32, local oscillations being impressed on the detector II by a local oscillator. The latter may be of any desired type; it is shown as comprising a triode 38, of the 6F7 type, which has a tunable circuit 34 connected between its control grid and cathode. Direct current blocking condenser 35 is connected between the high alternating potential side of circuit 34 and the grid, while a grid leak resistor 30 is connected between the grid and cathode.

The rotors of the variable tuning condensers of the amplifier 30, first detector II and oscillator 33 are mechanically coupled for uni-control tuning adjustment. The plate of tube 33 is reactively coupled, as at M, to the circuit N to produce local oscillations. Any well known method of transmitting the local oscillations to the mixer, or first detector, ll may be used. For example, the oscillations may be impressed upon the cathode of the detector 3| through coupling M1. The plate circuit of the mixer tube includes circuit 31 which is tuned to the operating I. F., for example 465 k. c. Any well known device may be electrically associated with the local oscillator to maintain the oscillator tuning tracking properly so as to keep the I F. value constant over the receiver tuning range. Such a device, for example, comprises condensers in series and shunt with the oscillator tuning condenser, and the condensers being properly chosen for the ,required function.

The I. F. energy in circuit 21 is amplified through amplifier 18 containing one, or more,

amplifier tubes; the input circuit 39 being coupled to circuit 31 and being tuned to the operatmg I. F. The amplified I. F. energy is impressed upon the network including the two rectifiers. This network is constructed substantially the same as that shown in Fig. 2. For this reason corresponding elements will be designated by the same numerals, but differentiated by prime notations. Thus, the primary circuit I is tuned to the I. F. of 465 k. c., and is magnetically coupled to the secondary tuned circuit l3, tuned to the same I. F. The high alternating potential side of the circuit I is connected through blocking condenser It to the midpoint of coil I4.

The condenser 2| has a low impedance at the operating I. F., and in general it is desirable that it be low at useful modulating frequencies. As shown the junction of resistors l9 and is connected to ground through condenser 23', and the audio and AVC voltages are taken off at this point. The junction point 40 is connected to the subsequent audio utilization network through an audio coupling reactance II. The AVC voltage is impressed on the stages'whose gain is to be regulated by a lead l2. The latter is connected to the signal grid circuits of the amplifier 3|, mixer 3| and I. F. amplifier 38 through pulsating current filter resistors 42. Those skilled in the art are fully aware of the manner of operating of the AVG circuit; this acts to regulate the gain of the controlled tubes in a sense to maintain the carrier amplitude at the circuit i3 substantially uniform despite carrier amplitude variations at collector A.

The differential direct current potential for the AFC function is taken from the cathode side of resistor, 19'. The lead 44 is connected, through filter "resistor sacrum t e cathode side" of'rfelsistor'lil' to thegrid 46 of 'thefreduency control tube 41, The latter maybe 'a 'pentode of thfe 6F? type and is"then"the"pentode section or the tube whose triode section is oscillator 33. The plate isconn'ected by lead 50 to the high alternating" potentialside ofjclrcuit' 34. Direct current potential of proper magnitude is supplied reshaping "from a positive potential source FB Direct "current blocking condenser 5l' is connected 1nser1es'w1tnine' -cou 34" of circuit 34. Tlie'ble'eder' 4 1"'i's connected between +3 and ground. Thcathbdes of oscillator 33 and'tube 41"are"-connected"to af tap 40' on bleeder 41'.

- Theinitialfbias for grid' '46 is provided bythc bleder section 49"; high frequency by-pass condenser 50' being shunted across section 49. The resistor 53 may have a magnitude'of the orderof20;000 ohms; and condenser iz may have a value of'0;0002"mfd. The grid 46 of tube41is connected to the junction of resistor 53 and condenser 52 through condenser 5'4. I

Considering the operation of the system shown in Fig.4, it"is' first'polnted out that the audio voltageic'omponent of the'detected I. F. current is taken ofl at 'point 40, because at resonance, when receiving {amplitude-modulated carrier waves, the audio, as well as'direct current, voltages across resistorsiil'f-and 20" will be equal and opposed. Hence, at' IifF. resonance there' will be no audio potentials between point 40' and ground. As far as' audio components" are-concerned the system actsexactly as though point 40' were grounded with -th"outputs of thet'wo diodes l1 and i8 actin in-"parallel; Therefore, the point 40, at the junction oi resistors l9} and 20', is a potent source'of "audio voltage to's'upply the following audio network (which may comprise one, or more st "ges oi audio amplification followed by a re- 'd1 i t :e'r)',' and no'other audio detector is necrect current voltage taken off between and ground has'the p'roperpolarity for U on. This'potential will bear the same ratio to the developed? audio voltages asis found in th'feonventi'onal diode detector AVC SYStBIll. Theiact that itrna'xiniizes at one side of reso' of no significance if the AFC network When the 'is'cut out of circuit, as I contiol'50; thepoint 40' is grounded, andthis cause the direct'potential at point 40 tof'maximize uniesonance; Ifit'i s necessary to xriaxirnize the AliQ- -D. C; impedance ratio it can be een that the D. impedance is equal to on'e 'halfthe resistance'o'f one of theseries" resistors? even. though the resistors are not in par'allel:as"far as direct curr ents arefconcerned;

The use"of'a"normally active control element in an automatically controlled frequency system willfnot permit the'carrl'er to depart sufficiently far from 'resonance to hazard the 'above 'facts'. Incidentally, it is to'bei noted that the transla-' t'ion" g'ainj of-the re'atl'fier'network (audio output divided by I. input) will begreater than with conventional circuits since the primary and one half *seconda'ry'volta'gesare added (vectorially-i,

but that the selectivity will approximate that of the primary circuit l alone.

Th'flir'ect current voltages taken off from saw -en 40' and'ground'are equaland opposed arr-1 resonance. Ii-Iowever, and as illustrated in' Fi when the energy applied to"cifrcu it l0 isof a 'frequencyofi resonance 'AFC voltage will r appea s eipenirbls 'Q coniwljiubei 41. The connections between the plate circuit of tube 41 and oscillator-circuit aresuch that a negative capacity is reflectedjacro'ss (the oscillator circuit. p

'1t"ca'n be seen that the resistor 53 and condenser 52 are in series across the oscillator tuned circuit. If the resistance of 53 islarge compared to the reactance of the eondensert z, currents through this series circuit will be substantially inphase with the voltage across-the oscillator tuned circuit. This current passing through the condenser 52 produces a voltage "across condenser 52 which lags the voltage across the oscillator tuned circuit by substantially 90. voltage is applied to the grid of" the control tube 41 which is preferably ofthe hlghmm high. plate imp'edance type. It can'then be seen that the plate current flowing in connection 50 to tube 41 will bejsub'stantially 90 ahead of the voltage across the oscillator tuned circuit. The current through the'tuning condenser of the oscillator tuned circuit lags the voltage across'that circuit about 90. Thus, any plate current flowing in connection 50 to tube 41 acts as though the current flowing in the variable tuning condenser had been decreased.

' In other words, the tube 41 produces a negative capacity effect on the oscillator circuit. The magnitude of this negative capacity is, of course, a function of the mutual conductance of tube 41. Thus; if the AFC voltage applied to the grid of tube 41 is positive thereby'overcoming some of the bias applied in the cathode circuit of that tube, its mutual conductance is increased The amount "of leading current flowing in connection 50"is thereby increased, 'which is the same as though the lagging current flowing through the variable: tuning condenser had been decreased. Thisin turn acts as though 'thatcondenser had been decreased invalue thereby causing the tuned frequency toiincrease. v

Assuming, now, that'a signal impressed on priniaryflcircuit I0 is approaching the I. F. value of itdk. c., but is less than'the'latter, and that point 40 will have a positive potential with respeqtflto ground. Theirequency departure may befdue to a shift in oscillator frequency towards a'lowe'r frequency, or due to tuning the receiver towards the high end of thetuningrange. Thus, the grid 46 becomespositive, and increases the gain of tube 41. Thisw ilfresult' in an increase cuits will automatically be made to increase, and approach, towardslthe desired 1. F. "value, The

reverse is true of the case'where the signal energy applied to circuit f0 is departing from I. F., and becominggreater in magnitude. The AFC action will commence as soon as a little 'of'the side band EIIEIgY Ol" a modulated carrier wave is applied to the primary circuit, I0.',

flit can be seen that ifjthe applied frequency is, for*'example,l'ower than the resonant frequency, the absolute magnitude of the alternating potentialsappIi'ed to one oithe diodes will be greater than those applied to the other diode. If the coi l l l' of the secondary circuit 13 were I reversed in its connectionto the diodes,'the diode formerly receiving the, greater .potential would now receivethe lesser potential. The polarity of the differential direct current voltage is then re, versed from the former casei. Itwill, therefore, be seen that 'the' polarity of point .40 can be made either positive or negative when the given oil-resonant frequency is applied. It is apparent that interchanging the diode connections at the ends of coil l4 produces the same result as reversing the phase of the coupling between coils i5 and I4. Accordingly, it will be seen that the polarity of thedirect currentvoltage produced at point 40, when a given off-resonant frequency is applied, depends on the phase of the coupling between coils l5 and It. In the circuit shown in Fig. 4 the coupling between the coils l4 and I5 is phased so that point 40' becomes positive with respect to ground when the applied signal is lower than the resonant frequency.

It is a relatively simple matter to service an- AFC system of the type shown in Fig. 4. Those skilled in the art are fully aware of the relative ease of aligning circuits i0 and II with the other I. F. circuits 31 and 30. Of course, the receiver may be of a type using a combined oscillatornrst detector network in place of independent circuits. In such a case, a pentagrid converter tube is used, as is well known.

Further, the frequency control circuit actuated by the AFC voltage may be replaced by other types of networks which will accomplish the desired results. For example, networks disclosed by C. Travis in application Serial No. 19,563, filed May 3, 1935 may be used. Again, if the choke 22' and condenser 23' are not used, a choke must be included in any external connection to the point 40.

The receiving system shown in Fig. 4 can be employed to receive frequency-modulated carrier waves. That is to say, the frequency variation response network shown embodied in the system of Fig. 4 can be utilized in connection with detecting frequency modulated waves. In Fig. 5 thereis only shown, in order to preserve simplicity of disclosure, the portion of the response network between the primary circuit i0 and the audio frequency network. It will be observed that the networks are substantially similar except for the following changes. In the first place the condenser 2l', when receiving frequency modulated waves, is given a magnitude which is sufilciently small to by-pass energy of intermediate frequency, but is small enough not to shunt the audio frequency currents. Furthermore, the condenser 23' is replaced by condenser 23", and the latter is connected in shunt with resistor is.

Lastly, the audio voltage component of the recti- -In the case of a receiver of frequency-modulated waves the AFC network will be of particular advantage since it is especially desirable in such reception to keep the oscillator circuit resonant to that frequency which will result in the operating I. I". when a desired station is tuned in. However, it will be observed that detection of the frequency-modulated waves is accomplished without the utilization of mistuned rectifier circuits, which are tuned to opposite sides of the operating carrier frequency, a method which has been employed heretofore. In the present detection arrangement of frequency-modulated waves the primary circuit I0 is tuned to the same carrier frequency as the secondary circuit ii. and the audio voltage at point 40' varies in polarity and magnitude in dependence upon the frequency departure corresponding to the modulation applied to the carrier wave.

Another use for the frequency variation response network shown herein is illustrated in Fig. 6. In this figure, there is shown a circuit arrangement for indicating in a visual manner when the carrier of s. transmitter departs from its predetermined operating frequency. Conventional, and well understood, circuit networks are conventionally represented in the circuit arrangement of Fig. 8. The numeral 00 represents the transmitter oscillator which may be considered as operating, for example, at 1000 k. e. The usual monitoring oscillator operates at a frequency of 1,000.5 k. 0.

Those. skilled in the art are fully aware of the method employed for monitoring transmitter oscillators so as to maintain them at the frequency of 1000 k. c. In order to indicate to the transmitting station operator when the oscillator frequency shifts, there is employed the arrangement of Fig. 6. In this arrangement the monitoring and transmitter oscillator energies are fed to a detector 6|, and there is filtered out from the detected output the 500 cycle energy resulting from the detection process. The numeral 02 designates any desired type of filter network which can pass energy of 500 cycles. The 500 cycle energy is impressed upon the frequency response network which comprises the primary winding 03. which is coupled as shown to the secondary winding. The primary and secondary windings may be of the iron core type, and are tuned to the 500' cycle frequency. The diode rectifier I has its anode connected to the high potential side of the secondary winding, and the cathode of rectifier is connected to.the low potential side of the secondary winding through a scrim path which includes resistor 60, resistor 01 and the cathode to anode space current path of diode rectifier 00. An alternating current by-pass condenser 00 is connected in shunt with resistors 60 and 01, and the visual indicating device 10 is connected in shunt -with condenser 00. Condenser 0! prevents resistor 61 from being short circifited. The condenser 03' could be placed, if desired, between ground and the low potential side of resistor 01.

The indicator [0 may be an ammeter which is properly calibrated to indicate departures as low as 0.1 cycle of! the 500 cycle input energy. It is possible to generate a voltage of 10 volts across the meter 10 to indicate a one cycle variation. It will be recognized that this indicating network is very sensitive, particularly at the frequency variations which are required in transmitter practice.

While I have indicated and d systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to be particular organisations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a frequency variation response network, a primary resonant circuit connected to a source of high frequency waves, said circuit being tuned to a desired frequency, a secondary resonant circuit including a coil tuned to the same frequency, means for reactively coupling said circuits, means for connecting the high alternating potential side severalq te ewt rim r-z 91 1 m. #0 he m min n thetaor the secondary circuitj'means'foi 'rectuym g the notent mt. the w end of. the secon a y e fc'uit c91 *tvitn-re pf t-td thfiow 'potehtlal side dii h 'lifi etrircfii n n a r'jfadd z h resulting dii"ect"current'voltages in opposition;

j z, Infa'radio receiving-system, a primary reso nan; circuit tunedtci ajdesired fciperating'signal' a i sq antc m tune m secondary circuits being react'iyeW coupled,

connections -between the primary" and sec ondary circuits 'such that two alternating currentf potentials of lilge polarity exist between the ends o f'the' secondary circuit and thelow poten} ua end of the primary, jdnef of isaid'potent'ials maximizing above f'the operatin frequency, and one maximizing below the latter, means'for rectify n' the antennas; mea s r 'a'fldit c th resulting" directjcurrent voltages in' Opposition, means for deriving fromsaid last-named means a'voltagecorresponding to the modulation of the received waves, and another voltage varying in magnitude with the waveamplitude.

3.1:: a radio receiver of the superheterodyne type and which employs a local oscillator, a rectifier network comprising. reactively coupled primary and secondary. circuits tuned to the operating intermediate frequency, means for rectifying the alternating current output of the secondary circuit,'connections between the primary and secondary' 'circuits such that the rectified signal potentials'have a magnitude and polarity determinedby the amount and" the sign ofthe differ-' ence betweenthe frequency of the coupled cir ujtsfandthe frequency .of the applied signal en-f erg'yQand means for utilizing'the differential direct current p otential of the rectified signal currents for' automatically regulating the frequency of the local oscillator;

n 4;. In combination with a source of modulated high frequency energy having a'predeterrnined carrier frequency, a frequency response network comprising a primary circuit which is tuned to the saidcarrier' frequency, a secondary circuit in-' 'cluding "a coil tuned to the same carrier frequeney; saidjprimary and secondary circuitsbein'g reactiyely coupled, means for connecting the high alternating potential side of the primary circuit to" the mid-point of thesecondary' circuit coil whereby two' alternating current potentials of like polarity existfbetween the opposite sides of the secondary circuit and the low potential sides of' the primary circuit, one of said alternating potentials manimizin'g above said carrier frequency, i andthe other maximizing" below the latter fre-;

quency, jafpath connecting the opposite sides of saidfsecondary' circuit, said path including a pair of dio defrectifiers' and a resistor connecting the cathodesof tlie"diodes, meansfor connecting the mid-point of said'resistorto said n'iid-=point on the econ ary circuit coll, andj rneans for main' taining the low alternatingpotential side ofsaid' ains e m e i r 5."In""'cornb ina 'i:on,' in superheterodyne' receiver of the type including a: first detector, local oscillator "and an: intermediate frequency amplifier,

ne .u t.. h samefixed Potential s.

least two'resouant circuits each including upled to each otner n arranged nf. heme? an l i s'be u le t ner 'to receiveenergy of intermediate er mm; each pf sa ieso ant ci'r;

, thelen e' ti e iniefinedia-ief 'r re'actiyely coupling s'aidcir fli's'ticiiuit being coupled-to tn l egp oipt of the s'econd respnant circuit "coil, means for rectifying the alternating potentialspf like, polarity atjthe twc ends of said second'jresonantcircuit with ,re-. spect to the'low potential fs'ld'e'efthe first circuit, means for adding the resul tin'g' direct current voltages in opposition,,meauseleetrically associated withthe local oscillator, for; determinin the frequency thereof, and additional means operativelyassociated with (said rectifying means and ,s'aid frequencydetermining meansfor applying said resultant direct curreut' voltage tofthe frequency determining meansfin response to a variation in the frequency magnitude of said n e efi uen ii en -4' I 6, In combination, in a superheterodyne [receiver of the type including'a flrst detector, local oscillato w t e fimdlata jreqi en y a p fier, at least two resonantcircults' each including a coil coupled to each other andfarrangfed in cascad'e',,the first of said circuits being'coupled to said amplifier to receive energy of intermediate frequency therefrom, each"of"s 'ai'd resonant circuits ,bein g tuned to the operating intermediate frequency, means for" reactively coupling said circuits, the high alternating potential side of said first circuit being coupled to the mid-point of the second resonant circuit coil, means for rectifying the a1te'rnating' potentials of like polarity at the two ends of said second resonant circuit with respect tothe low potential side of the first circuit, means for adding the resulting direct current voltages in opposition, means electrically associated with the local oscillator for determining the frequency thereof, additional i means opcratively associated with said rectifying said circui'ts' being coupled to said amplifier to receive energy of intermediate frequency therefrom, each of said resonant circuits being tuned to the operating intermediate frequency, means for reactively coupling said circuits, the high alternating potentialside of said firstcircuit being' coupled to the'mid-point of the second resonant, circuit, means for rectifying the potentials atthetwo'e'nds of saidsecond resonant circuit with respect 'to the low potential end of the first circuit', 'rneans for adding the resultingdirect current'voltages in opposition, means electrically associated with the local: oscillator for determining the frequency thereof; and additional meansoperatively associated with said rectifying means and said frequency determininglrneans for apply; ing said resultant direct current voltage to the frequency determining means responsei toa variation in the freguncy rnagnitude of said intermediate frequency energy,and auto atic vol umebontrol means for impressing the direct, our} rentjvolt'age derived'fio'n r diate frequencyen'ergy telthelinput Circuit of at leastone of the transmiss'io n the first of said 'coupled resonan n mee s i mt I Fi lh firequency oscillations of a predetermined frequency, a second source diflering therefrom by a predetermined frequency amount, means for detecting the diflerence frequency of the oscillations from said two sources, a primary circuit, tuned to said difierence frequency, coupled to the output of said detector, a secondary circuit, tuned to said difference frequency, reactively coupled to said primary circuit, the potentials across said coupled primary and secondary circuits differing in phase when energy of said difference frequency is impressed on the primary circuit, and means, responsive to a phase angle change solely between said potentials when the.

high frequency energy applied to the primary circuit departs from resonance with said coupled circuits, for varying the frequency of one of said sources in a sense to correct said departure.

9. In a superheterodyne receiver adapted to receive frequency-modulated carrier waves, and which receiver includes, in addition to. a first detector, local oscillator network including a frequency determining element, intermediate frequency amplifier and audio frequency transmission network, a detection network comprising a primary circuit, tuned to the intermediate frequency, coupled to the output of the intermediate frequency amplifier, a secondary circuit, tuned to the intermediate frequency, reactively coupled to said primary circuit, a path of low impedance to the intermediate frequency energy connected between the high alternating potential side of said primary circuit and the mid-point of the secondary circuit, a diode rectifier having its anode connected to one side of said secondary circuit, a second diode rectifier having its anode connected to the other side of said secondary circuit, a resistive impedance connected in series between the cathodes of said two diodes, means for connecting the mid-point of said resistive impedance to said first named mid-point, an audio frequency transmission path connected to one side of said resistive impedance, the opposite side of said impedance being at a relatively fixed potential, means for applying the direct current potential developed across the entire resistive impedance to said frequency determining element in said local oscillator network for automatically varying the frequency of the oscillator when the energy impressed on said primary circuit departs from said intermediate frequency value.

10. In a superheterodyne receiver which in-' diate frequency, means for deriving a direct cur-.

rent voltage from said intermediate frequency energy, when the frequency of. the intermediate energy departs from the predetermined value, an electron discharge tube having its space current path connected in shunt across said tunable oscillation circuit, an electrode in the space current path of said last named tube connected to have said derived direct current voltage impressed on it, a reactive path, including a resistor in series with a condenser, connected in shunt with said space current path and said tunable oscillation circuit, and means for impressing the alternating current potential developed across said reactive path condenser upon the electrode disposed in said space current path whereby variation of the direct current voltage of said electrode will result in a variation of the effective reactance in said tunable oscillation circuit. I

11. In combination with a resonant circuit tuned to a desired wave frequency, a second resonant circuit tuned to the same frequency, said circuits being reactively coupled, a source of waves coupled to the first circuit, a pair of diode rectiilers, one of the diodes having its anode connected to a point of high alternating potential on the second circuit, the other diode having its anode connected to a point of relatively low alternating potential on the second circuit, a resistive impedance connecting the cathodes of said diodes, means establishing a point of said second circuit intermediate said two points at the potential of the high alternating potential side of the first circuit, means esta an intermediate point of said impedance at said first intermediate point potential, and means for utilizing direct current voltage developed across said impedance by the rectification action of the diodes. 12. In combination with a first circuit tuned to a desired frequency, a second circuit tuned to the same frequency, said circuits being reactively coupled, a source of alternating current coupled to the first circuit, a pair of diode rectiflers, one of the diodes having its anode connected to a point of high alternating potential on the second circuit, the other diode having its anode connected to a point of relatively low alternating potential on the second circuit, a resistive impedance connecting the cathode: of said diodes, means establishing a point of said second circuit intermediate said two points at the potential of the high alternating potential side of the first circuit, means establishing an intermediate point of said impedance at said first intermediate point potential, and a visual current indicator means for utilizing direct current voltage developed across said impedance by the rectiiication action of the diodes.

13. In a superheterodyne receiver of the type provided with a local oscillator having a tank circuit tuned to a desired frequency, a network utilizing oscillations from the oscillator and producing a beat frequency, a first resonant circuit tuned to the beat frequency, a second resonant circuit tuned to the same beat frequency, said beat circuits being reactively coupled, a pair of diode rectifiers, one of the diodes having its anode connected to a point of high alternating potential on the second circuit, the other diode having its anode connected to a point of relatively low alternating potential on the second circuit, a resistive impedance connecting the oathodes of said diodes, means establishing a point of said second circuit intermediate said two points at the potential of the high alternating potential side .of the first circuit, means establishing an intermediate point of said impedance at said first intermediate point potential, and means, responsive to the direct current voltage developed across said impedance by the rectification action of the diodes, for automatically ad- Justing the tank circuit frequency.

14. In combination with a first circuit tuned to a desired intermediate frequency, a second circuit tuned to the same frequency, said circuits being reactively coupled, a source of intermediate frequency energy coupled to the first circuit, a pair of diode rectifiers, one of the diodes having its anode connected to a point of high alternating potential on the second circuit,the other diodes having its anode connected to a point of relatively low alternating potential on the second circuit, a resistor connecting the cathodes of said diodes, means including a condenser establishing a point of said second circuit intermediate said two points at the potential of the high alternating potential side of the first circuit, a condenser of low impedance to intermediate frequency current connected in shunt with said impedance, means establishing an intermediate point of said impedance at said first intermediate point potential, and means for utilizing direct current voltage developed across said impedance by the rectification action of the diodes.

15. In combination with a resonant circuit tuned to a desired wave frequency, a second resonant circuit tuned to the same frequency, said circuits being reactively coupled, a source of waves including an amplifier coupled to the first circuit, a pair of diode rectifiers, one of the diodes having its anode connected to apoint of high alternating potential on the second circuit, the other diode having its anode connected to a point of relati Iely low alternating potential on the second circuit, a pair of series resistors of equal value connecting the cathodes of said diodes, means establishing a point of said second circuit intermediate said two points at the potential of the high alternating potential side of the first circuit, means establishing the junction point of said resistors at said first intermediate point potential, and means for utilizing direct current voltage developed across said resistors by the rectification action of the diodes.

16. In combination with a first circuit transmitting waves of a desired frequency, a second circuit tuned to the said frequency, said circuits being reactively coupled, a source of waves coupled to the first circuit, a pair of diode rectiflers, one of the diodes having its anode connected to a point of high alternating potential on the second circuit, the other diode having its anode connected to a point of relatively low alternating potential on the second circuit, a resistive impedance connecting the cathodes of said diodes,

means establishing a point of said second circuit intermediate said two points at the potential of the high alternating potential side of the first circuit, means establishing an intermediate point of said impedance at said first intermediate point potential, means for utilizing direct current voltage developed across the entire impedance by the rectification action of the diodes. and additional means for utilizing direct current voltage developed between the impedance intermediate point and one end of said impedance.

17. In combination with a resonant circuit tuned to a desired wave frequency, a second resonant circuit tuned to the same frequency, said circuits being reactively coupled, a source of waves coupled to the first circuit, a pair of diode rectifiers, one of the diodes having its anode connected to a point of high alternating potential on the second circuit, the other diode having its anode connected to a point of relatively low alternating potential on the second circuit, a resistive impedance connecting the cathodes of said diodes, means establishing a point of said second circuit intermediate said two points at the potential of the high alternating potential side of the first circuit, means establishing an intermediate point of said impedance at said first intermediate point potential, means for utilizing direct current voltage developed across said impedance by the rectification action of the diodes, said last means comprising a network provided with a frequency determining circuit, and a direct current voltage connection being connected between one end of the impedance and said frequency determining circuit for frequency adjustment of the latter.

18. In combination with a resonant circuit tuned to a desired wave frequency, a second resocoupled to the first circuit, a pair of diode rectifiers, one of the diodes having its anode connected to the high alternating potential side of the second circuit, the other diode having its anode connected tothe low alternating potential side of the second circuit, a resistor connecting the cathodes of saiddiodes, one end of the resistor being at ground potential, means establishing the mid-point of said second circuit at the potential of the high alternating potential side of the first circuit, means connecting the mid-point of said resistor to said first mid-point, and. a direct current voltage path connected to the ungrounded end of the resistor for utilizing direct current voltage developed across the resistor by the rectification action of the diodes.

STUART W. SEELEY. 

